Giant switchable non thermally-activated conduction in 180° domain walls in tetragonal Pb(Zr,Ti)O3

Conductive domain walls in ferroelectrics offer a promising concept of nanoelectronic circuits with 2D domain-wall channels playing roles of memristors or synoptic interconnections. However, domain wall conduction remains challenging to control and pA-range currents typically measured on individual walls are too low for single-channel devices. Charged domain walls show higher conductivity, but are generally unstable and difficult to create. Here, we show highly conductive and stable channels on ubiquitous 180° domain walls in the archetypical ferroelectric, tetragonal Pb(Zr,Ti)O3. These electrically erasable/rewritable channels show currents of tens of nanoamperes (200 to 400 nA/μm) at voltages ≤2 V and metallic-like non thermally-activated transport properties down to 4 K, as confirmed by nanoscopic mapping. The domain structure analysis and phase-field simulations reveal complex switching dynamics, in which the extraordinary conductivity in strained Pb(Zr,Ti)O3 films is explained by an interplay between ferroelastic a- and c-domains. This work demonstrates the potential of accessible and stable arrangements of nominally uncharged and electrically switchable domain walls for nanoelectronics.


Supplementary Table 1 | Comparison of domain wall conductance values in literature.
In the table (next page), DW conductance values are compared together with the respective readout bias for different materials and DW configurations. Necessary writing/erasing biases to create the (charged) DWs are given as well as the used readout method and other remarks.
Notably, significant conduction (>1nA) is observed only for strongly charged DWs prepared using a special poling procedure or formed transiently under voltage.  It is important to note that probing the 180°-DWs, which are created through polarization reversal, in the negative voltage range is only applicable in a very narrow range due to the asymmetric phase-loop. The coercive voltage on the negative side is around -1V. By applying a stronger negative bias, the polarized domain is modified and the DW is pushed away from its original position, making probing of its conductive properties impossible. This behaviour is actually used as an advantage in our configuration, the asymmetric phase-loops opens up the possibility to use the DW conduction on the positive side in a wider range up to ~+4V without destroying or moving the DW. consecutive IV curves were performed on an artificially created 180°-DW using the AFM-tip as a top electrode. Clearly, the current readout is not destroying the conductivity measured through the DW nor displacing the DW to an extend that the conduction significantly drops.
This confirms that the measured conduction is not associated with the volatile polarization switching currents. Because in the case of polarization switching current, the measured conduction would be associated with the displacement of the DW and would drastically decrease once the DW is pushed away. b, cAFM image at 4 K. The sample bias was 6V and similar current values were observed in comparison to a. c, IV curves measured on a pristine 90°-DW at 4K. As already seen in the cAFM scans, a weak tens of pA conduction is maintained even at very low temperatures. fields. Before and after topography images of the studied area in Fig. 3a-d. The effect of using a high electric-field for poling (>7V) is observed in the formation of surface particles. The PFMphase images serve to visualize the area, which is affected by the high electric fields. These particles remained even after scanning in contact mode. By using lower electric field, fewer particles could be produced but at the same time, the uniformity of the poling is reduced.

Supplementary Note 1 | Fowler-Nordheim formalism and fitting parameters
The I-V curve in Fig. 1o was fitted using classic Fowler-Nordheim formula which describes tunneling through the triangular potential barrier as a function of applied electric field F: where are electron effective tunneling masses for the tip and PZT. S, e, and h are the effective tunneling area, electron charge and Plank constant, respectively. In order to convert the applied voltage V to the electric field F we used a model of spherical tip of radius R 11 , which yields a convenient approximation applicable within the relevant length scale: F=V/R.
The fit in Fig.1o was obtained for the potential barrier = 0.8 , = , = 3 (where is the free electron mass), effective tip radius of 2nm and S=10nm 2 .